Strength envelope for UU triaxial (Q Test)

[GeotechPE]

Recently conducted UU triaxial test on remolded lean clay with sand at confining pressures of 7, 14 and 28 psi. Two specimens at each pressure (6 mohr cicles). Samples were remolded to 95 percent standard proctor at plus 2 percent of optimum MC. Initial saturation is about 75 percent. Samples were not backsaturated. Sheared at 1 percent per minute.

With UU tests on saturated clay I would expect phi = zero or very close to zero and C = some value. If I draw a strength envelope to the mohr circles (which fits the six circles very well) I get a phi = 21 degrees and c = 7 psi.

I am trying to explain the significant slope of the strengh envelope with the following:

1) Samples not saturated, therefore strength more resembles drained condition.
2) Too fast a shear rate for material or shear rate incorrectly set too high.
3) Sample overconsolidated by standard proctor remolding. I would expect this to occur to some degree but not to this magnitude.

Any one gone down this road before? Also does anyone have any references / papers on the amount of shear strength introduced by standard compaction test effort?

Thanks.

 

[BigH]

Did you consolidate the samples at the different confining pressures or did you immediately test after reaching the confining pressure?

 

[GeotechPE]

BigH -- The samples were not consolidated.

 

[GeoPaveTraffic]

I would say the lack of consolidation is the main issue followed by the saturation.

If you run a conslidation on one of the compacted samples you will find the minimum confining pressure needed to have a nomally consolidated sample. If you then increased the pressures from there you would likely get the "expected" results. However, you may find that the required pressures are so high that you cannot run the tests and that the proposed use of the material will not generate those pressures.

Also, if you consolidate the samples at above the Pc, the amount of saturation will be much higher.

 

[MRM]

Same thought as GeoPaveTraffic, different way to say it:

Since the specimens were not consolidated at the various confining pressures (basic UU test procedure), with a saturation less than 100%, and compacted specimens (each compacted to a similar degree), I would expect to see nothing other that a curve similar to the one you described. If you are able to, run 2 more tests at greater confining pressures; say one at about 50psi and one at about 80psi or greater. No drainage/consolidation allowed. You'll find that the slope will flatten out to the characteristic "phi=0" curve.

As an aside: although I understand how and why you're using phi and c to describe the "shape" of the curve, keep in mind that it could be misleading to many folks when describing UU strength. I like to avoid that whole mess by ultimately calling the strength parameter gained from this testing "su," or undrained shear strength.

 

[GeotechPE]

Thanks for the responses GeopaveTraffic and MRM.

I had one additioal specimen tested at a confining pressure of 42 psi. This mohr circle was much larger than the 28 psi circle. The Pc point must be at an even higher pressure. I am in the process of getting approval to run the consoliation test at my employers cost, just to learn more.

I have not prevously appreciated the shear strength gained by standard compaction. Impressive.

MRM with respect to "As an aside". I feel the term undrained shear strength is misleading. The specimen was tested in an undrained test method but the result measured is a drained condition due to low saturation (75%) and OCR condition.

Thanks again.

 

[BigH]

Check out the following and associated references - might be of some help:

http://www.ejge.com/1999/Ppr9902/Ppr9902.htm

 

[GeotechPE]

Thanks BigH. I will read the paper tonight.

 

[TBrandon]

There are a couple of other papers that you might want to read. One is Seed, Mitchell and Chan from the 1960 Boulder Shear Strength Conference. And the best paper might be Kulhawy, Duncan, and Seed (1969). I think that it's a UCB report, and it's the best paper about UU strengths of compacted clays.

 

[BigH]

TBrandon - full reference on Kulhawy?

 

[TBrandon]

"Finite Element Analyses of Stresses and Movements in Embankments During Construction," (F. H. Kulhawy, J. M. Duncan, and H. Bolton Seed), Office of Research Services, Report No. TE 69-4, University of California, November, 1969. 169 pages.

"Strength, Stress-Strain and Bulk Modulus Parameters for Finite Element Analyses of Stresses and Movements in Soil Masses," (J. M. Duncan, and P. Byrne, K. S. Wong and P. Mabry), Geotechnical Engineering Report No. UCB/GT/80-01, University of California, Berkeley, August, 1980. 77 pages.